This invention relates to a polarization control system for use in an optical communication system for bidirectionally transmitting optical signal beams. In general, an optical communication system comprises an optical fiber having first and second bifer ends. The optical fiber is, for example, from 20 to 100 km long and has a birefringence which is inevitably subjected at various points of the length to variations due to the ambient temperature of the optical fiber, external pressure supplied to the optical fiber, or both. It is therefore unavoidable that a signal beam has a polarization state or plane which undergoes fluctuations from time to time while transmitted through the optical fiber.
In the optical communication system, a first or first-end transmitter device and a first or first-end receiver device are connected to the first fiber end. A second or second-end transmitter device and a second or second-end receiver device are connected to the second fiber end. Each of the first and the second receiver devices is operable with a local beam having a local polarization state to produce a detection signal having a power level.
The first transmitter device is for generating a first signal beam and for supplying the first signal beam to the first fiber end with a first original polarization state. Transmitted through the optical fiber, the first signal beam arrives at the second fiber end as a first arriving beam to be received by the second receiver device. Due to the fluctuations in the polarization state, the first arriving beam has a first variable polarization state. Consequently, the second receiver device can not always have a best receiver sensitivity.
The second transmitter device is for generating a second signal beam and for supplying the second signal beam to the second fiber end with a second original polarization state. Transmitted through the optical fiber, the second signal beam arrives at the first fiber end as a second arriving beam to be received by the first receiver device. The second arriving beam has a second variable polarization state. As a result, the first receiver device can not always have a best receiver sensitivity.
It is possible to avoid occurrence of the fluctuations in the polarization state and to always achieve best receiver sensitivities if the optical fiber is a polarization maintaining optical fiber. The polarization maintaining optical fiber is, however, very expensive. It is therefore usual to use a single-mode optical fiber and a polarization control system in order to suppress the fluctuations.
The polarization control system may be operable according either to automatic polarization control or to polarization diversity. When the polarization diversity is resorted to, each receiver device must have a complicated structure. Moreover, the receiver sensitivity becomes poor. It is therefore preferred to use the automatic polarization control for simplicity of each receiver device and for achievement of a high receiver sensitivity.
An excellent polarization control system is disclosed in U.S. patent application Ser. No. 527,470 filed May 23, 1990, by the present applicants based on two patent applications which were filed in Japan and given Patent Application Nos. 130,510 and 130,511 of 1989. The polarization control system is for use in a bidirectional optical communication system of the type described and comprises a single polarization control device. The United States patent application will be referred to as a prior patent application in the following.
In the manner known in the art, the polarization control device comprises an optical fiber which is called a control fiber in the prior patent application and has a device and a fiber end as called in the prior patent application. It should be noted in connection with the prior patent application and the instant patent application that the control fiber is a part of the single-mode optical fiber. The device end is therefore one of the above-mentioned first and second fiber ends and may be the first fiber end without loss of generality.
According to the prior patent application, the single polarization control device is for collectively controlling the first and the second variable polarization states. A little more in detail, a control circuit is connected to the first receiver device to be supplied with the detection signal and controls the polarization control device so as to keep the power level maximum.
In the prior patent application, polarization dispersion is discussed. The polarization dispersion occurs due to the birefringence of the optical fiber either when the first and the second signal beams are single signal beams and have a finite frequency difference or when at least one of the first and the second signal beams is the frequency division multiplexed signal beam in which individual component signal beams have frequency differences. The single polarization control device is therefore effective only when the frequency difference is within a certain allowable range. As a consequence, the number of collectively polarization controllable channels is restricted. Furthermore, the polarization dispersion becomes serious when the optical fiber is long. The polarization dispersion therefore restricts a communication possible distance or length.